Would be useful for wp_upload_dir to be able to return without date sub directories

Description

I'm writing a plugin which needs to cache certain web readable files (CSS & JS, etc). The obvious writable directory is wp-content/uploads/, however to work out the filesystem path and web path for this dir is not easy, because wp_upload_dir always returns with the date based subdirs. I could remove the subdirs from the paths afterwards, but this seems needlessly hard.

The attached patch adds an additional argument to wp_upload_dir which requires the function to return the dirs without the date subdirs. The argument defaults to false to maintain backwards compatibility.